This invention relates to injection molding, and more particularly to an apparatus and method for removing internally threaded plastic parts, such as container closures, from the mold core of an injection mold.
Injection molding generally involves introducing molten plastic under pressure into a space defined between a core part and a cavity part of an injection mold. The molten plastic injected into the space is allowed to coot and thereby solidify to form a xe2x80x9cpartxe2x80x9d, after which the core and cavity parts are separated. The part generally shrinks a small amount upon cooling and remains on the core part of the mold from which it must be removed or xe2x80x9cstrippedxe2x80x9d.
The removal of internally threaded parts from a threaded core has in the past presented a problem. Parts with shallow threads may sometimes be forced off the core using a stripper plate. However, deeper threads would be damaged by any effort to force them off the core with a stripper plate, and therefore are preferably removed by rotation or xe2x80x9cunthreadingxe2x80x9d of the part from the core.
A number of devices and methods are known for simultaneously unthreading and pushing a threaded part from a mold core. One example of such a device is described in U.S. Pat. No. 5,383,780 (McCready et al.). The McCready apparatus includes a rotatable stripper ring which surrounds the mold core. After the part is formed and the mold is opened by separating the cavity part from the mold core, the stripper ring is rotated and simultaneously lifted relative to the core, simultaneously unthreading and pushing the threaded part away from the core. It is also known in the prior art to provide a rotatable core which is unthreaded from the part. Examples of patents which utilize a rotatable core are U.S. Pat. No. 2,306,205 (Crosman, Jr.) and Japanese Patent Application No. 62-264923.
Known devices for stripping threaded parts from a mold core have numerous disadvantages, such as increasing the size and complexity of the molding apparatus and reducing the speed of the molding process. Increased complexity can result in increased equipment and maintenance costs, while increased size can limit the number of mold levels which may be accommodated in a molding apparatus.
Accordingly, an improved method and apparatus for removing threaded parts from plastic injection molds is required.
The present invention overcomes the above-described problems of the prior art by providing an apparatus and a method for removing threaded parts from plastic injection molds. In the apparatus of the present invention, the cavity part of the mold is rotatable relative to the core part, such that simultaneous rotation of the cavity and opening of the mold results in unthreading of the part from the core.
The apparatus for forming threaded molded parts according to the present invention comprises a first mold plate and a second mold plate. The apparatus has a mold closed position in which a mold is defined comprising the mold core and the mold cavity, and a mold open position in which the mold core and mold cavity are separated by a sufficient distance so as to permit removal of the part from the apparatus. The first mold plate (also referred to herein as the mold core plate) carries the mold core, the core having an external threaded surface. The second mold plate (also referred to herein as the mold cavity plate) carries the mold cavity, which is rotatable about a mold axis parallel to the direction of relative movement of the mold plates.
The mold cavity is rotationally coupled to a mold cavity rotator which is preferably driven by means of a rack. The rotation of the mold cavity is timed with the mold opening step such that the threaded part is unthreaded from the core as the mold plates are separated. The rotator for the mold cavity preferably comprises a rotatable shaft and a radially extending drive pinion. The shaft has gear teeth and is driven by the rack. Thus, movement of the rack results in rotation of the drive pinion, which meshes with a pinion extending radially about the mold cavity, causing rotation of the mold cavity.
The apparatus according to the invention preferably also includes a third mold plate (also referred to herein as the mold stripper plate) which carries a stripper ring. The stripper ring is axially movable relative to the core and is used to eject the part from the core after it is completely unthreaded. Preferably, the stripper ring is provided with at least one air passage through which pressurized air can be passed to assist in ejecting the unthreaded part from the core.
Since the part is unthreaded by the rotating mold cavity, it is desirable to prevent the part from rotating relative to the mold cavity during unthreading. Plastic closures for containers typically have an outer serrated surface to assist in unscrewing the closure from the container. These serrations on the part correspond to serrations provided on an inner axial surface of the mold cavity and advantageously provide sufficient resistance to relative rotation of the part and the mold cavity during unthreading.
In order to ensure that the part becomes separated from the mold cavity after it is unthreaded from the core, the apparatus preferably includes means to hold the part to the core until after the part is substantially completely unthreaded and the mold cavity is withdrawn from the part. In a preferred aspect of the invention, a negative pressure is created in a space which is formed between the part and the mold core as the part is unthreaded, and preferably near the end of the unthreading operation. This negative pressure is preferably applied through an air passage extending through the core.
The apparatus and method for forming threaded molded parts have a number of advantages over the prior art. Firstly, the mechanism for rotating the mold cavity is relatively simple, which can reduce equipment and maintenance costs and improve reliability. Secondly, the mechanism for rotating the mold cavity is relatively compact, permitting it to be housed in a single mold plate. This may permit a greater number of mold levels to be stacked in the press of an injection molding apparatus. Thirdly, in the apparatus and method of the invention, the part is unthreaded from the core as the mold plates are separated, eliminating the need for a separate mold opening step, thus improving the speed of the molding process.